1. Field of the Invention
The present invention relates to a fixing device in an image forming apparatus for fixing onto a sheet paper, by heating and pressurizing, a toner image formed in the image forming apparatus using an electrophotographic process such as a copy machine, printer or facsimile, and a control method thereof.
2. Description of the Background
As a fixing device incorporated in an image forming apparatus using an electrophotographic process such as a copy machine, printer or facsimile, there is a fixing device that fixes a toner image on a sheet paper by heating and pressurizing. In such a fixing device, a sheet paper is inserted between a pair of rollers including a heat roller and a press roller, or in a nipping part formed between similar belts, and a toner image is fixed thereon by being heated and pressurized. Meanwhile, a full-color toner image formed by superimposed toner images of plural colors has a toner layer thickness that is several times larger than that of a monochrome toner image.
Therefore, when the temperature of the heat roller is raised to melt the thick toner layer, only the surface of the toner layer melts excessively. If only the surface of the toner layer melts excessively, a high-temperature offset may occur. Moreover, for a color image, a difference in the way of melting of the toner layer causes a color difference in the image and predetermined image quality cannot be acquired. That is, there is a need to melt the color toner layer for preventing the color difference.
Thus, in a conventional, as a color image fixing device, there is a device that heats a heat roller and also heats a press roller supporting the rear side of a sheet paper. This conventional fixing device can evenly melt a toner layer by heating the toner layer from both above and below. Also, the conventional fixing device can prevent a high-temperature offset of the toner.
However, also in such a conventional fixing device, heating control of the heat roller and the press roller is only on-off control of the heating source by application of predetermined power for each operation mode. Therefore, the heat roller and the press roller tend to have a temperature ripple and this temperature ripple may cause a color difference in the fixed image.
Meanwhile, recently, energy saving is demanded in an image forming apparatus. In the image forming apparatus, the power consumption by the fixing device is large and the energy saving in the fixing device largely affects the energy saving in the entire apparatus. Conventionally, when the power source of the image forming apparatus is turned on, power is also supplied to the fixing device at the time of warming up and at the time of copying. Moreover, the fixing device is supplied with power also when it is in the standby state for the next copy after being ready. This supply of power in the standby state reduces the user's waiting time until the start of copying, and it is highly convenient. However, power is also consumed in the standby state as well as while copying is being carried out. Moreover, in the conventional color fixing device, in which both the heat roller and the press roller are heated, the power consumption in the standby state may increase.
Thus, in fixing a color toner image with a thick toner layer, by evenly melting the toner image, it is possible to provide uniform image quality without generating a color difference in the fixed image. Moreover, a control method of the fixing device in the image forming apparatus is desired which enables energy saving without losing the convenience for the user.